b 1. Field of the Invention
The present invention relates to a variable speed pulley device which is provided between a vehicle engine and an auxiliary machinery and works as a part of a belt type nonstep variable speed gear mechanism which comprises a pair of variable speed pulley devices.
2. Description of the Prior Art
A variable speed pulley device has been disclosed in a Japanese Utility Model Application No. 62-145949. This application, "a drive mechanism of a mechanical type supercharger," discloses a drive mechanism in which a belt type nonstep variable speed gear mechanism is provided between an engine and a mechanical type supercharger (which corresponds to an auxiliary machinery). In this variable speed gear mechanism, a flange interval of a variable speed pulley device is changed according to engine speed changes by a cam which receives centrifugal force from a flyweight, whereby a pitch radius of a belt can be regulated. In this mechanism, rotational frequency changes of the supercharger may be controlled over a broad range of engine speeds (i.e. from a low-middle rotational frequency range to a high rotational frequency range).
FIG. 1 illustrates a variable speed pulley device 201 corresponding to the above-mentioned variable speed pulley device. This variable speed pulley device is provided on the mechanical type supercharger side (i.e. auxiliary machinery side) in the belt type nonstep variable speed gear mechanism. The variable speed gear mechanism is provided between the vehicle engine and the mechanical type supercharger. Namely, the other variable speed pulley device is provided on the engine side. The variable speed pulley device employed in the mechanical type supercharger side comprises a pair of cams 209. One of the cams is formed between a fixed cam member 203 which is fixedly mounted on an axle (which is not shown in FIG. 1) and a flyweight 207. The other cam is formed between said flyweight 207 and a movable flange 205 which is fixedly mounted on said axle. The variable speed pulley device on the mechanical type supercharger side and the variable speed pulley device on the engine side (which is not shown in FIG. 1) are coupled by a belt 211. Tension is given to the belt 211 by a spring which is provided between flanges of the engine side variable speed pulley device (not shown).
In the above described art, when centrifugal force of the flyweight 207 is applied to the cams 209, the movable flange 205 moves against the belt tension toward a fixed flange which is not shown in FIG. 1 but would be located on the right hand side of FIG. 1. When the movable flange 205 moves in this way, belt pitch radii of both the engine side variable speed pulley device and the supercharger side variable speed pulley device become changed, causing changes in the supercharger side rotational frequency which is conveyed from the engine side. Namely, the nonstep variable speed gear mechanism is to minimize the rotational frequency changes when conveying the engine driving force to the supercharger even if the engine speed changes radically. That is, in this kind of variable speed gear mechanism, it is required that engine driving force be reduced in accordance with a demanded property of an auxiliary machinery such as a supercharger.
With regard to the variable speed pulley device in the above-described prior art, the surfaces of the cams are flat. Namely, an angle which is made between faces of each cam is invariable. This angle will be shown and described as a cam angle .theta. hereinafter.
FIG. 2 is a line graph describing a relationship between rotational frequencies in the drive pulley device (i.e. on the engine side) and rotational frequencies in the driven pulley device (i.e. on the auxiliary machinery side) in a case which a small can angle .theta. is predetermined. When the engine device increases, the rotational frequency of the driven pulley side becomes substantially increased as shown by slope 213. A large thrust force is created in the cam 209 due to the small cam angle .theta.. After a while, a speed changing point 215 appears in a low rotational frequency range where the thrust force is balanced with the belt tension. As the engine speed increases further, the movable flange 205 starts to move, whereby the engine speed is conveyed toward the auxiliary machinery while the engine speed is reduced. In this situation, the rotational frequency of the auxiliary machinery is almost fixedly maintained within a low rotational frequency range as shown by slope 217. As the engine speed increases even more, the movable flange 205 reaches its moving limit, whereby a speed reducing function of the variable speed gear mechanism stops and the rotational frequency of the driven pulley device starts to increase again as shown by slope 221.
As described above, when a small cam angle .theta. is predetermined, the rotational frequencies in the auxiliary machinery at and after the speed changing point are not high enough for the supercharger. Also, a high frequency range for the supercharger appears after a certain period of the low rotational frequency range thereof, wherein rotational frequencies of the driven pulley device substantially increase once again according to the rotational frequencies of the drive pulley device. Namely, after the low rotational frequency range is passed, the variable speed gear mechanism can no longer keep up with the high engine speed.
The above-described problems may be improved by strengthening the belt tension. However, in this situation there is a limit to how much the belt tension can be strengthened because rotational resistance caused by friction between the belt and a pulley may be increased resulting in a shorter life span for the belt.
On the other hand, a predetermined big cam angle .theta. causes the thrust force of the cam 209 t be reduced, whereby a speed changing point 225 reaches to a high level a shown by slope 223 in FIG. 3. However, the rotational frequencies in the supercharger side remain at increased levels when the speed range is reduced, due to the small size of the thrust force, as shown by slope 227. Thus, a big cam angle .theta. also cannot fixedly maintain the rotational frequency in the supercharger side.
As described above, it is very difficult for the prior variable speed gear mechanism using the prior variable speed pulley device to satisfy necessary properties for use with auxiliary machineries, for example, high speed changing points and/or fixedly maintained rotational frequencies in the auxiliary machinery side.